Method for manufacturing oriented silicon steel product with high magnetic-flux density

ABSTRACT

A method for manufacturing an oriented silicon steel product with high magnetic-flux density comprises the following procedures: 1) smelting and casting, wherein the oriented silicon steel is composed of, by weight, 0.035˜0.065% of C, 2.9˜4.0% of Si, 0.05˜0.20% of Mn, 0.005˜0.01% of S, 0.015˜0.035% of Al, 0.004˜0.009% of N, 0.005˜0.090% of Sn, 0.200˜0.800% of Nb, the rest being Fe; and after being smelted, molten steel is secondarily refined and continuous casted into steel slabs; 2) hot rolling; 3) normalizing; 4) cold rolling; 5) decarburization annealing; 6) MgO coating; 7) high temperature annealing: said sheets are firstly heated to 700˜900° C. and then secondarily heated to 1200° C. at temperature rise rate of 9˜17° C./hr and maintained at 1200° C. for 20 hr; 8) coating an insulation layer. According to the present invention, steel sheets can be fully nitrided during high temperature annealing, which can ensure a secondary re-crystallization to take place perfectly, thereby, the oriented silicon steel sheets with high magnetic-flux density can be achieved. The present invention solves the problem of nitriding that is encountered in production of high-magnetic-induction oriented silicon steel by the technique to heat steel slabs to a lower temperature.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing oriented silicon steel sheet, and particularly, to a method for manufacturing oriented silicon steel sheet with a high magnetic-flux density.

BACKGROUND OF THE INVENTION

The conventional process for manufacturing oriented silicon steel with high magnetic-flux density is as follows. After being smelted in a convertor or electric furnace, molten steel is secondarily refined and alloyed, and then continuous-casted into steel slabs. Its basic chemical compositions are: Si 2.5˜4.5%, C 0.06˜0.10%, Mn 0.03˜0.1, S 0.012˜0.050, Al 0.02˜0.05%, N 0.003˜0.012. Some composition systems further contain one or more of the elements Cu, Mo, Sb, B, Bi, etc. The rest is Fe and unavoidable impure inclusions. A steel slab is heated to a temperature over 1350 in a special furnace and maintained at the temperature for more than 45 min so as to make the advantageous inclusions MnS or AlN fully solid-dissolved, then rolled into steel plates with a finishing temperature up to over 950° C.; and then a plate is cooled rapidly to below 500° C. by jetting water, thereafter, coiled to be coil-shaped. Subsequently, during normalization, fine and dispersed second phase particles, namely depressant, separate out of silicon steel body. After being normalized, the hot rolled steel plates are pickled and removed of oxidized scale, and then cold-rolled into sheets of the thickness of finished steel sheet product. A cold rolled sheet is decarburization annealed and coated with an anneal insulator (main composition is MgO). The carbon in the sheet is decarburized to the extent as not to influence the magnetic property of the finished steel sheet product (generally it shall be below 30 ppm); during being high temperature annealed, the steel sheet generates physical and chemical changes such as secondary re-crystallization, formation of bottom layer of magnesium silicate and purification (elements S, N, etc., harmful to magnetic property, are eliminated from the steel sheet), and is made to be a highly-oriented, low-iron-loss and high-magnetic-induction silicon steel sheet; finally, after being coated with insulation layer and tension-annealed, the silicon steel sheet is made to be commercially available oriented silicon steel sheet product.

The shortcomings of the above manufacturing process is that the heating temperature must be up to 1400° C. in order to have the depressant fully solid-dissolved. This is the upmost level of a conventional heating furnace. In addition, because of the high heating temperature, burning loss is big and the heating furnace needs to be frequently mended, thus resulting in a low utilization. Also, energy consumption is high. Moreover, the hot rolled coil-shaped plate often has larger edge cracks, which may cause difficulty in the subsequent cold rolling procedure, and result in a low yield rate, unsatisfactory magnetic property B₈ of the finished product, and higher manufacture cost.

In view of the problem above, both domestic and foreign researchers have carried out a lot of research with the aim to reduce the heating temperature of oriented silicon steel. The research can be categorized into two types. One is to heat a steel slab to a temperature within the range of 1250˜1320° C. and to use AlN and Cu as a depressant. The other is to heat a slab to a temperature within the range of 1100˜1250° C. and to acquire depression capability by employing a depressant which is formed by nitriding after decarburization.

Nowadays, there has been a rapid development in the technique for heating a steel slab at a lower temperature. For example, in US patent U.S. Pat. No. 5,049,205 and Japanese patent publication JPA 1993-112827, a steel slab is heated to a temperature not higher than 1200° C. and rolled into plates. In the finishing cold rolling procedure, a plate is rolled into sheets with a great rolling compression ratio of 80%, and the rolled steel sheet is continuous nitriding treated by use of ammonia after they are decarburization annealed in order to obtain highly oriented secondarily re-crystallized grains. In this technique, however, because the depressing effect is obtained by a depressant which is generated by nitriding of the rolled steel sheet after it is decarburized, it is very difficult, in actual control, to avoid the problems such that the steel sheet will have severely oxidized surfaces and it is hard to be nitrided evenly. Therefore, it will lead to the difficulty for obtained-type depressant to generate and evenly distribute in the steel sheet, and thus it will affect depressing effect and evenness of secondarily re-crystallized grains, and finally result in uneven magnetic property of the finished silicon steel sheet product.

Chinese Patent CN 200510110899 describes a new process, where steel slabs are heated at a temperature not higher than 1200° C., and the cold rolled steel sheets, which have been rolled to the thickness of the finished product, are nitrided prior to decarburized annealing. In this process, however, it is necessary to strictly control the dew point during nitriding, and there will occur a new problem that decarburization becomes more difficult.

Recently, Korean Patent KR 2002074312 disclosed that steel slabs are heated to a temperature not higher than 1200° C., and rolled sheets are decarburized and nitrided simultaneously. Although the difficulties in post-rolling decarburization and post-rolling nitriding can be solved, however, uneven nitriding is still unavoidable and thus will give rise to uneven magnetic property of the finished silicon steel sheet product, and manufacture cost may be higher.

Adding element Nb is also proposed. For example, in Japanese patents JP 6025747 and JP 6073454, Nb of 0.02˜0.20% is added in the compositions of smelted steel. It is aimed at generating niobium carbide and niobium nitride and thereby to fine the re-crystallized texture, improving grain distribution and collective texture of the decarburization annealed steel sheets, taking the niobium carbide and niobium nitride as an auxiliary depressant to depress the growing up of the normal grains during high temperature annealing, and thus improving the magnetic property of silicon steel sheets. However, a problem with the patents is that the steel slabs must be heated to a very high temperature in order to parse out niobium nitride before hot rolling, and this will certainly lead to a greater burning loss, higher energy consumption, a lower ratio of the finished product, and a higher manufacture cost.

According to Japanese patent JP51106622 and US patent U.S. Pat. No. 4,171,994, nitrates of Al, Fe, Mg and Zn are added into a separant MgO. It aims at making them decomposed during high temperature anneal and thus releasing nitrogen oxide so as to nitride steel sheets. However, the nitrogen oxide and oxygen out of the decomposed nitrates may lead to an explosion risk in practical production.

According to Japanese patent JP52039520 and American patent U.S. Pat. No. 4,010,050, sulfanilic acid is added in separant MgO. It is aimed at making sulfanilic acid decompose in high temperature and thus release nitrides for nitriding. However, being a organic substance, sulfanilic acid will decompose at a lower temperature (about 205° C.), the nitrogen released at so low temperature is hard to make steel sheet nitrided.

According to Japanese patents JP 61096080 and JP62004811, nitriding of steel sheets during high temperature anneal is realized by adding nitrides of Mn and Si. However, a problem with this method lies in that these nitrides have a high thermostability. Therefore, they can not be decomposed effectively and quickly. In order to meet nitriding requirement, it is necessary to prolong the period of high temperature annealling or to increase the quantity of those nitrides.

With regard to temperature rise rate during high temperature annealing, Japanese patents JP 54040227 and JP200119751 put forward that oriented silicon steels with high magnetic-flux density can be obtained by reducing temperature rise rate in the course of high temperature annealing. However, simply reducing temperature rise rate may result in a greatly reduced production rate.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for manufacturing an oriented silicon steel product with high magnetic-flux density, which solves the difficulty in nitriding for manufacturing oriented silicon steel sheet with high magnetic-flux density where steel slabs are heated at a lower temperature. The present invention efficiently ensures safe and stable operation and a long life of smelting furnaces by a technique of heating at a lower temperature. In the manufacture process, oriented silicon steel sheets can be fully nitrided during high temperature annealing, which can ensure secondary re-crystallization to take place perfectly, and thereby, the oriented silicon steel sheets with high magnetic-flux density and premium magnetic property can be achieved.

The invention adopts the following technical solution.

During smelting, a certain amount of Nb is added in the compositions of oriented silicon steel so as to make oriented silicon steel sheet be easy to be nitrided because the nitrogen content in steel is crucial in deciding whether the magnetic property of the finished oriented silicon steel sheet product meet specifications. Some nitrates are added in MgO separant and the MgO separant added with nitrates is applied on the surfaces of the steel sheets before the sheets are high temperature annealed. During high temperature annealing, the nitrates decompose and release nitrogen which can make the steel sheets fully nitrided. The temperature rise rate during high temperature annealing is regulated according to the Nb content, N content prior to secondary heating and the beginning temperature of secondary heating, thus ensuring secondary re-crystallization to take place perfectly, and thereby, the oriented silicon steel sheets with high magnetic-flux density and premium magnetic property can be achieved.

Specifically, a method for manufacturing an oriented silicon steel product with high magnetic-flux density is provided according to the present invention. The method includes the following procedures:

1) Smelting and Casting

The oriented silicon steel is composed of, by weight, 0.035˜0.065% of C, 2.9˜4.0% of Si, 0.05˜0.20% of Mn, 0.005˜0.01% of S, 0.015˜0.035% of Al, 0.004˜0.009% of N, 0.005˜0.090% of Sn, 0.200˜0.800% of Nb, the rest is Fe and unavoidable inclusions. After being smelted, molten steel is secondarily refined and then casted into steel slabs.

2) Hot Rolling

Said steel slabs are heated in a heating furnace to 1090˜1200° C., and then, are hot rolled into steel plates at a beginning temperature of 1180° C. and are finished with the hot rolling step at a finishing temperature of 860° C., said steel plates are cooled by laminar flow of water to below 650° C. and then coiled into coiled-shape plates.

3) Normalization

A coiled-shape plate is normalized at the normalization temperature of 1050˜1180° C. for 1˜20 sec and then at the normalization temperature of 850˜950° C. for 30˜200 sec, and thereafter, is cooled down at a cooling rate of 10˜60° C./sec;

4) Cold Rolling

After being normalized, the steel plate is cold rolled into steel sheets with the thickness of the finished oriented silicon steel sheet product at a rolling compression ratio not less than 75%;

5) Decarburization Annealing

A steel sheet is heated to the temperature of 800˜860° C. at a temperature rise rate of 15˜35° C./sec and maintained at the temperature for 90˜160 sec for being decarburized, herein only decarburization must be carried out because nitriding will take place during high temperature annealing;

6) MgO Coating

After being decarburized, said steel sheets are covered with a coating which is composed of, by weight, 0.1˜10% of NH₄Cl and 0.5˜30% of P₃N₅, and MgO as rest wherein MgO is a main component.

7) High Temperature Annealing

After being coated with the isolator, the steel sheet is firstly heated to a temperature of 700˜900° C., and then secondarily heated to 1200° C. at temperature rise rate V_(secondary temperature rise) of 9˜17° C./hr and maintained at 1200° C. for 20 hr for being purification annealed and nitrided;

8) Coating an Insulation Layer

After being high temperature annealed, the surfaces of the steel sheet is coated with an insulation layer, and then is tension and leveling annealed, and finally becomes the oriented silicon steel sheet with high magnetic-flux density and premium magnetic property.

According to the present invention, a certain amount of Nb is added into the silicon steel. There two reasons of doing this. The first reason is that the oriented silicon steel with Nb in its compositions is much easier to be nitrided, this is because the d sublayer of sub-outer spheres of the atom of Nb is unsaturated with electrons and so Nb is much easier to change into nitrides than Fe and Mn, and nitride of Nb is very stable. The second reason is that the N atoms, which penetrate into steel sheets during high temperature annealing, can bond with Al to generate main depressant AlN which is necessary to obtain oriented silicon steel sheet with high magnetic-flux density, and also can be combined into Nb₂N and NbN. These nitrides of Nb can be an auxiliary depressant and can intensify depressing effect on growth of normal crystal grains. In general, this solution is very advantageous to improve the magnetic property of oriented silicon steel sheet.

According to the present invention, a certain amount of NH₄Cl and P₃N₅ is added into a liquid MgO coating. The intention of doing this is to use decomposition of the two nitrides during high temperature annealing to realize nitriding of silicon steel sheets, and thereby to replace the nitriding which will take place by virtue of decomposition of ammonia during decarburization anneal, the greatest benefit of this solution is to ensure the steel sheets to be nitrided evenly. The reason of selecting NH₄Cl and P₃N₅ as nitriding material which will decompose at high temperature is that NH₄Cl will decompose at 330˜340° C. and P₃N₅ will decompose at 760° C. or so. The decomposition of the two different nitrides at different temperatures ensures to evenly release active atoms of nitrogen in a relatively long time in the procedure of high temperature annealing, this is advantageous to nitriding of the steel sheets and to maintaining N content therein to be within standard limits of 200˜250 ppm.

According to the present invention, the temperature rise rate for the secondary heating during high temperature annealing is controlled to ensure the finished oriented silicon steel sheet product to attain premium magnetic property by setting a proper secondary temperature rise rate. This is because the course of secondary temperature rise for high temperature annealing covers the whole temperature range of secondary re-crystallization. Therefore, a proper temperature rise rate can ensure the Gauss grains which grow during the secondary re-crystallization to have a much better orientation (deviation angle <3°) and magnetic property.

According to the present invention, the relatively low temperature rise rate during high temperature annealing can refine the secondary re-crystallization and ensure the finished steel sheet product to have a better magnetic property. This is because gradual coarsening and decomposition of AlN as well as the secondary re-crystallization can take place simultaneously during secondarily heating for high temperature annealing, and so the depressing effect disappears simultaneously. If the temperature rises too quickly within this temperature range, it will result in such a case that the depressant has decomposed and lost its effect before the secondary re-crystallization has not yet finished. As is known, imperfect secondary re-crystallization will bring about poor magnetic property of the finished oriented silicon steel sheet product.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described in detail in conjunction with following embodiments.

First Embodiment

Material steel of oriented silicon steel sheet with the chemical compositions shown in Table 1 is smelted and casted into slabs. The slabs with different chemical compositions are heated to the temperature of 1155° C. in a heating furnace and maintained at the temperature for 1.5 hours, and then hot rolled into plates of 2.3 mm thickness at a beginning temperature of 1062° C. and finishing temperature of 937° C. The hot rolled plates are normalized in two phases: at 1120° C. for 15 sec and at 870° C. for 150 sec ((1120° C.×15 sec)+(870° C.×150 sec)), and then cooled down at the rate of −15° C./sec. After being pickled, the hot rolled plates are cold rolled to coil-shaped steel sheets with the thickness 0.30 mm of the finished steel sheet product, and then in sequence, the cold rolled coil-shaped sheets are heated at temperature rise rate of 25° C./sec to decarburization temperature of 820° C. and maintained at the temperature for 140 sec for being decarburization annealed; applied and covered with a thick layer of a separant which contains MgO as the main component and NH₄Cl of 4.5% and P₃N₅ of 15%; heated to 800° C. for being high temperature annealed and getting nitrogen content b before being secondarily heated; secondarily heated to temperature of 1200° C. and maintained at the temperature for 20 hours for being purification annealed. After being uncoiled to steel sheets of some length, the sheets are applied with an insulation coating layer and then are tension and leveling annealed. The nitrogen content b prior to secondary heating and the magnetic property of the finished steel sheet product are both shown in Table 1.

TABLE 1 effect of chemical compositions on nitrogen content before secondary heating and magnetic property N content before secondary heating Instance C % SI % Mn % S % AL % N % Sn % Nb % (ppm) B₈ T P17/50 W/kg 1 0.035 3.2 0.20 0.010 0.015 0.009 0.090 0.20 202 1.92 0.97 2 0.041 2.9 0.10 0.005 0.025 0.006 0.070 0.36 211 1.92 0.99 3 0.052 4.0 0.05 0.008 0.035 0.004 0.005 0.64 234 1.93 0.97 4 0.065 3.5 0.15 0.012 0.022 0.007 0.035 0.80 244 1.92 0.98 Comparison 0.046 3.0 0.08 0.006 0.028 0.008 0.072 0.18 173 1.87 1.11 Examples 1 Comparison 0.053 3.5 0.15 0.011 0.019 0.006 0.014 0.84 292 1.86 1.12 Examples 2

As can be seen from Table 1, the selection of various chemical compositions according to the embodiment is in consistence to the standard specification (of smelting and casting) in the production procedures of the present invention. However, the selection of component Nb in the comparison examples is not within the standard limits of 0.200˜0.800, therefore, the amount of N measured before secondary heating is not within the standard limits of 200˜250 ppm, and finally causes the finished oriented silicon steel sheet product to have a larger iron loss (P_(17/50)) and a poor magnetic property (B₈).

Second Embodiment

The oriented silicon steel slabs is composed of (by weight percent) the following elements: C 0.05%, Si 3.25%, Mn 0.15%, S 0.009%, Al 0.032%, N 0.005%, Sn 0.02%, Nb 0.5%, the rest is Fe and unavoidable impurities. The slabs are heated to the temperature of 1155° C. in a heating furnace and maintained at the temperature for 1.5 hours, and then hot rolled into plates of 2.3 mm thickness at a beginning temperature of 1080° C. and finishing temperature of 910° C. The hot rolled plates are normalized in two phases: at 1110° C. for 10 sec and at 910° C. for 120 sec ((1110° C.×15 sec)+(910° C.×120 sec)), and then cooled down at the rate of −35° C./sec. After being pickled, the hot rolled plates are cold rolled into coil-shaped sheets with the thickness 0.30 mm of the finished steel sheet product, and then in sequence, the cold rolled coil-shaped sheets are heated to the decarburization temperature of 840° C. at temperature rise rate of 25° C./sec and maintained at the temperature for 130 sec for being decarburization annealed; applied and covered with a thick layer of a separant which contains MgO as the main component and NH₄Cl and P₃N₅ of certain small contents; heated to 800° C. for being high temperature annealed and getting nitrogen content b before being secondary heated; secondarily heated to temperature of 1200° C. and maintained at the temperature for 20 hours for being purification annealed. After being uncoiled to steel sheets of some length, the sheets are coated with an insulation layer and then are tension and leveling annealed. The nitrogen content b prior to secondary heating and the magnetic property of the finished steel sheet product are both shown in Table 2.

TABLE 2 effect of the contents of NH₄Cl and P₃N₅ on nitrogen content before secondary heating and magnetic property N content before secondary heating Instance NH₄Cl % P3N5 % (ppm) B₈ T P17/50 W/kg 1 0.1 3.9 198 1.92 0.99 2 1.2 11.3 210 1.91 1.00 3 3.6 20.8 231 1.92 0.98 3 6.4 0.5 206 1.92 0.97 4 8.3 6.6 221 1.92 1.00 5 10 12.8 222 1.93 0.96 6 2.4 19.5 234 1.92 0.98 7 5.5 26.4 252 1.91 0.99 8 1.9 30 243 1.93 0.96 Comparison 6.4 0.4 178 1.87 1.10 Examples 1 Comparison 2.4 30.2 268 1.88 1.06 Examples 2 10.5 30.5 283 1.83 1.16

As can be seen from Table 2, the selection of NH₄Cl and P₃N₅ according to the embodiment is in consistence to the standard ranges of 0.1˜10% and 0.5˜30% (of MgO coating) in the production procedures of the present invention. Contrarily, in the selection of NH₄Cl and P₃N₅ in the comparison examples, whatever one is not within the standard limits causes the content of N measured before secondary heating to be not within the standard limits of 200˜250 ppm, and finally causes the finished oriented silicon steel sheet product to have a larger iron loss (P_(17/50)) and a poor magnetic property (B₈).

Third Embodiment

The oriented silicon steel slabs is composed of the following components: C 0.05%, Si 3.25%, Mn 0.15%, S 0.009%, Al 0.032%, N 0.005%, Sn 0.02%, Nb (a)0.2˜0.8%, the rest is Fe and unavoidable inclusions. The slabs are heated to the temperature of 1155° C. in a heating furnace and maintained at the temperature for 2.5 hours, and then are hot rolled into plates of 2.3 mm thickness at a beginning temperature of 1050° C. and finishing temperature of 865° C. The hot rolled plates are normalized in two phases: at 1120° C. for 15 sec and at 900° C. for 120 sec ((1120° C.×15 sec)+(900° C.×120 sec)), and then cooled down at the rate of −25° C./sec. After being pickled, the hot rolled plates are cold rolled to coil-shaped sheets with the thickness 0.30 mm of the finished steel sheet product, and then in sequence, the cold rolled coil-shaped sheets are heated to the decarburization temperature of 850° C. at temperature rise rate of 25° C./sec and maintained at the temperature for 115 sec for being decarburization annealed; applied and covered with a thick layer of a separant which contains MgO as the main component and NH₄Cl of 7.5% and P₃N₅ of 12.5%; heated to 700˜900° C. as beginning temperature (c) of the secondary heating in high temperature annealing and for getting nitrogen content (b) before being secondary heated; heated to the temperature of 1200° C. at a certain temperature rise rate (V) and maintained at the temperature for 20 hours for being purification annealed. After being uncoiled to steel sheets of some length, the sheets are applied with an insulation coating layer and are tension and leveling annealed. The data of the third embodiment are shown in Table 3.

TABLE 3 effect of different processes of both normalization and nitriding on the magnetic property of the finished silicon steel sheet product beginning N content temperature theoretically actual before of calculated secondary secondary secondary secondary heating difference magnetic nb heating heating heating rate rate (° C./hr) property (%) (ppm) (° C.) (° C./hr) (° C./hr) V_(upper limit) − P_(17/50) Instance a b c V_(upper limit) V_(actual) V_(actual) B₈ T w/kg 1 0.20 186 700 17.9 16 1.9 1.90 1.00 2 0.20 184 800 14.3 14 0.3 1.90 0.98 3 0.20 189 900 10.5 9 1.5 1.91 1.01 4 0.40 204 720 18.2 17 1.2 1.92 0.96 5 0.40 207 810 14.8 14 0.8 1.91 0.99 6 0.40 211 880 12.2 12 0.2 1.93 0.93 7 0.60 231 750 18.0 17 1 1.93 0.95 8 0.60 229 850 14.3 14 0.3 1.92 0.99 9 0.80 248 780 17.9 15 2.9 1.91 1.00 10  0.80 252 860 14.8 12 2.8 1.92 0.96 Comparison 0.20 186 700 17.9 19 −1.1 1.85 1.07 Examples 1 0.20 184 800 14.3 15 −0.7 1.86 1.09 2 0.20 189 900 10.5 12 −1.5 1.85 1.08 3 0.40 204 720 18.2 20 −1.8 1.85 1.12 4 0.40 207 810 14.8 16 −1.2 1.86 1.09 5 0.40 211 880 12.2 14 −1.8 1.84 1.15 6 0.60 231 750 18.0 19 −1 1.85 1.12 7 0.60 229 850 14.3 15 −0.7 1.87 1.14 8 0.80 248 780 17.9 19 −1.1 1.86 1.10 9 0.80 252 860 14.8 17 −2.2 1.84 1.12 10  0.20 184 800 14.3 15 −0.7 1.86 1.09

As can be seen in Table 3, in the case where Nb contents (a), N contents before secondary heating (b) and the beginning temperatures of secondary heating (c) all are the same, and also in the case where the actual secondary temperature rise rates in the embodiments are 9˜17° C./hr and the differences between the theoretically calculated values and the actual values are positive, the magnetic properties of the finished silicon steel sheet products in both the embodiments and the comparison examples are better. If the conditions are reversed, the cases of the comparative objects are adverse, and therefore, the electromagnetic properties of the comparative objects are poor.

To manufacture the oriented silicon steel sheet by heating steel slabs at a lower temperature has the advantages such as long life of heating furnace, lower energy consumption and lower manufacture cost. However, there exist the problems of uneven decarburization and uneven nitriding in the subsequent procedures and difficulties in efficient regulation and control in the course of production for a long time. Such cases have had influence on the depressing effect in some parts of a steel sheet or the whole sheet, and thus results in imperfect secondary re-crystallization and inconsistent magnetic property of the finished product.

In conclusion, the present invention provides a new method for manufacturing an oriented silicon steel sheet with high magnetic-flux density based on the procedure of heating steel slabs at a lower temperature. According to the method of the present invention, the above-mentioned problems are all effectively solved. The method is characterized in that the steel sheets can be easily nitrided during high temperature annealing by adding a certain amount of Nb in molten steel; the steel sheets can be evenly nitrided during high temperature annealing by adding some nitrides into the separant MgO and letting them decomposing during high temperature annealing; in the course of high temperature annealing, the temperature rise rate can be controlled according to Nb content, N content and the beginning temperature of secondary heating so as to ensure completion of a good secondary re-crystallization course. All these solutions ensure the achievement of oriented silicon steel sheet with high magnetic-flux density and premium magnetic property. 

1. A method for manufacturing an oriented silicon steel product with high magnetic-flux density comprising the following steps: casting oriented silicon steel into steel slabs after the silicon steel is smelted and secondarily refined, wherein the oriented silicon steel is composed of, by weight, 0.035˜0.065% of C, 2.9˜4.0% of Si, 0.05˜0.20% of Mn, 0.005˜0.01% of S, 0.015˜0.035% of Al, 0.004˜0.009% of N, 0.005˜0.090% of Sn, 0.200˜0.800% of Nb, the rest being Fe and unavoidable inclusions; heating said steel slabs in a heating furnace to 1090˜1200° C.; hot rolling said steel slabs into steel plates at a beginning temperature of 1180° C. and finishing hot rolling said steel slabs at a finishing temperature of 860° C.; cooling said steel plates using laminar flow of water to below 650° C.; coiling said steel plates into coiled-shape plates; normalizing said coiled-shape plates at a normalization temperature of 1050˜1180° C. for 1˜20 sec and at a normalization temperature of 850˜950° C. for 30˜200 sec, and then immediately thereafter cooling down the plates at a cooling rate of 10˜60° C./sec; wherein after being normalized, cold rolling said steel plates into steel sheets with a thickness of a finished sheet at a rolling compression ratio not less than 75%; decarburizing said steel sheets to a temperature of 800˜860° C. at a temperature rise rate of 15˜35° C./sec and maintained at the temperature for 90˜160 sec; coating said steel sheets with a coating including, by weight, 0.1˜10% of NH₄Cl and 0.5˜30% of P₃N₅, and MgO as the rest component where the MgO is a main component; heating said steel sheets to a temperature of 700˜900° C., and then heating said steel sheets to 1200° C. at a secondary temperature rise rate of 9˜17° C./hr and maintained at 1200° C. for 20 hr; and coating surfaces of the steel sheets with an insulation layer. 